A know induction heating apparatus shown in FIG. 6, comprises an AC power source 1; a rectifier 2 for commutating AC power from AC power source 1 into DC power; an inverter circuit 3 having two insulated gate bipolar transistors (IGBTs) 11 and 12 as switching elements for converting DC power from rectifier 2 into a high frequency AC power; a heating coil 4 connected to output terminals of inverter circuit 3; and a control circuit 5 for producing drive signals D1, D2 to turn IGBTs 11 and 12 in inverter circuit 3 on and off, and thereby, supplies high frequency AC power to heating coil 4.
AC power source 1 comprises a commercial AC power supply, and rectifier 2 comprises diodes 24 in bridge connection for commutating AC power from AC power source 1, and a capacitor 23 for bypassing or smoothing switched current from diodes 24. IGBTs 11, 12 comprise first and second IGBTs 11 and 12 connected in series between positive and negative terminals of rectifier 2, and reflux diodes 21 and 22 each connected to first and second IGBTs 11 and 12 in the adverse direction. A series circuit of a resonance capacitor 25 and heating coil 4 is connected in parallel to second IGBT 12. Heating coil 4 is driven by high frequency AC power to produce high frequency magnetic flux in magnetic coupling with a heated object made of metal such as iron for induction heating of the heated object.
Control circuit 5 comprises a drive circuit 7 for producing drive signals D1 and D2 to IGBTs 11 and 12, a resonance waveform detector 6 for detecting high frequency AC waveform such as electric current, voltage or power through heating coil 4 to produce detection signals DS1 in response to high frequency AC waveform through heating coil 4, a phase comparator 8 for comparing phases in detection signals DS1 from resonance waveform detector 6 and in drive signals D1 from drive circuit 7 to produce an adjusting signal PH of the level corresponding to the phase difference between detection signals DS1 and drive signals D1, an integrating circuit 57 for converting adjusting signal PH from phase comparator 8 into an averaged DC voltage, and an impedance regulator 40 for producing an impedance corresponding to output level from integrating circuit 57 to vary oscillation frequency in drive signals D1 from drive circuit 7. Not shown but, drive circuit 7 comprises an oscillator which may produce oscillation outputs for driving IGBTs 11 and 12. Otherwise, drive circuit 7 may comprise a driver or drivers for shaping output signals from oscillator into a waveform suitable for driving of IGBTs 11 and 12. Accordingly, drive signals D1 from drive circuit 7 represent output signals from oscillator or drivers. For example, oscillator may comprise a well-known variable frequency (VF) converter, and phase comparator 8 may comprise a well-known digital phase comparator.
Resonance waveform detector 6 comprises a detective transformer 26 for picking out resonance current flowing through heating coil 4 or resonance capacitor 25, a resistor 27 connected in series to detective transformer 26 for converting resonance current picked out by detective transformer 26 into voltage of the level corresponding to resonance current, and a limiter 61 having a resistor 28 and diodes 29 and 30. A junction of resistor 28 and diode 29 provides an output terminal of resonance waveform detector 6 connected to a first input terminal IN1 of phase comparator 8 through a capacitor 38 for removing DC component from output signals of limiter 61 so that resonance waveform detector 6 produces detection signals DS1 to phase comparator 8. In this way, resonance waveform detector 6 detects resonance current of high frequency AC power supplied from inverter circuit 3 to heating coil 4 to produce detection signals DS1 corresponding to high frequency AC waveform. Since inverter circuit 3 furnishes heating coil 4 with high frequency resonance current, detective transformer 26 produces detection signals of widely fluctuating level, however, limiter 61 serves to limit voltage value of detection signal DS1 by resonance waveform detector 6 below a predetermined voltage level. Drive circuit 7 produces drive signals D1 to a second input terminal IN2 of phase comparator 8 through a resistor 47.
Integrating circuit 57 comprises first and second dividing resistors 41 and 42 connected between output terminal of phase comparator 8 and ground, and a capacitor 43 connected between a junction of first and second dividing resistors 41 and 42 and ground. An impedance regulator 40 comprises a field-effect transistor (FET) 44 as a variable impedance element, a resistor 45 connected between source terminal of FET 44 and ground, and third and fourth dividing resistors 37 and 46 connected between an input terminal of drive circuit 7 and ground. FET 44 has a control or gate terminal connected to a junction of first and second dividing resistors 41 and 42 and capacitor 43, and a drain terminal connected to a junction of third and fourth dividing resistors 37 and 46.
Resonance waveform detector 6 delivers detection signals DS1 to a first input terminal IN1 of phase comparator 8, and drive circuit 7 provides drive signals D1 for a second input terminal IN2 of phase comparator 8. As shown in FIG. 7, detection signals DS1 from resonance waveform detector 6 are supplied to first terminal IN1 of phase comparator 8 earlier than drive signals D1 from drive circuit 7, indicating that detection signals DS1 from resonance waveform detector 6 precede in phase drive signals D1 from drive circuit 7. Under the preceding condition in phase of detection signals DS1, at the moment detection signals DS1 of high voltage level from resonance waveform detector 6 reach first input terminal IN1 of phase comparator 8, drive signals D1 of low voltage level from drive circuit 7 come to IN2 of phase comparator 8 which therefore produces an adjusting signal PH of high voltage level shown in FIG. 7(c). Then, when phase comparator 8 receives detection signal DS1 of high voltage level from resonance waveform detector 6 and drive signal D1 of high voltage level from drive circuit 7, it produces an adjusting signal PH of intermediate voltage level M. Thereafter, phase comparator 8 maintains to produce adjusting signal PH of intermediate level M, even though either or both of detection signal DS1 from resonance waveform detector 6 and drive signal D1 from drive circuit 7 are shifted to low voltage level.
To the contrary, drive signals D1 from drive circuit 7 reach phase comparator 8 earlier than detection signals DS1 from resonance waveform detector 6 under the preceding condition in phase of drive signals D1, indicating that drive signals D1 from drive circuit 7 precede in phase detection signals DS1 from resonance waveform detector 6. Under the preceding condition in phase of drive signals D1, at the moment drive signals of high voltage level from drive circuit 7 reach second input terminal IN2 of phase comparator 8, detection signals DS1 of low voltage level from resonance waveform detector 6 come to IN1 of phase comparator 8 which therefore produces an adjusting signal PH of low voltage level L shown in FIG. 7(c). Subsequently, when both of resonance waveform detector 6 and drive circuit 7 produce detection signals DS1 and drive signals of high voltage level to phase comparator 8, it produces an adjusting signal PH of intermediate voltage level M. Next to this, phase comparator 8 keeps adjusting signal PH of intermediate voltage level M even though either or both of detection signal DS1 from resonance waveform detector 6 and drive signal D1 from drive circuit 7 are shifted to low voltage level.
Specifically, when phase of detection signal DS1 from resonance waveform detector 6 to first input terminal IN1 advances ahead of phase of drive signal D1 from drive circuit 7 to second input terminal IN2, phase comparator 8 produces an adjusting signal PH of high voltage level H in intermediate voltage level M. Otherwise, when phase of detection signal DS1 from resonance waveform detector 6 to first input terminal IN1 lags behind phase of drive signal D1 from drive circuit 7 to second input terminal IN2, phase comparator 8 produces an adjusting signal PH of low voltage level L in intermediate voltage level M. Further, phase comparator 8 continues to produce an adjusting signal PH of intermediate level M when detection signal DS1 from resonance waveform detector 6 and drive signal D1 from drive circuit 7 are simultaneously on the high or low voltage level.
Adjusting signal PH from phase comparator 8 causes electric current to flow through first dividing resistor 41 of integrating circuit 57 into capacitor 43 which serves to average adjusting signals PH from phase comparator 8. Voltage in capacitor 43 of varied level by electrically charging or discharging is applied to gate terminal of FET 44. When high level voltage in capacitor 43 by charging is applied to gate terminal of FET 44, it is turned on to increase electric current through FET 44, thus reducing impedance in impedance regulator 40. Adversely, when low level voltage in capacitor 43 by discharging is applied to gate terminal of FET 44, it diminishes electric current therethrough to increase impedance in impedance regulator 40.
In operation, two drive signals D1 and D2 from drive circuit 7 are alternately applied to each base terminal of a pair of IGBTs 11 and 12 to alternately turn IGBTs 11 and 12 on and off. Drive signals D1 and D2 forwarded from drive circuit 7 do not simultaneously turn IGBTs 11 and 12 on, however, do turn one of IGBTs 11 and 12 on, while turning the other off. Moreover, a dead time is provided for simultaneously turning IGBTs 11 and 12 off after turning one off and before turning the other on. When first IGBT 11 is turned on while second IGBT 12 is kept off, electric current from AC power source 1 through rectifier 2, first IGBT 11, heating coil 4 and resonance capacitor 25 to rectifier 2 to activate heating coil 4 and electrically charge resonance capacitor 25. Adversely, when second IGBT 12 is turned on while first IGBT 11 is kept off, resonance current flows from resonance capacitor 25 through heating coil 4 and IGBT 12 to resonance capacitor 25, electrically discharging resonance capacitor 25. In this way, IGBTs 11 and 12 are alternately turned on and off to perform high frequency induction heating of heating coil 4.
During the operation of heating coil 4, detective transformer 26 detects resonance current passing between heating coil 4 and resonance capacitor 25 to cause limiter 61 to produce detection signal DS1 to first input terminal IN1 of phase comparator 8. Concurrently, drive circuit 7 produces a drive signal D1 to second input terminal IN2 of phase comparator 8 through resistor 47. As mentioned in connection with FIG. 7, when phase of detection signal DS1 moves forward faster than phase of drive signal D1 moves late so that detection signal DS1 is on high voltage level and drive signal D1 is on low voltage level, phase comparator 8 generates an adjusting signal PH of high voltage level H. To the contrary, when phase of drive signal D1 advances faster than phase of detection signal DS1 moves late so that drive signal D1 is on high voltage level, and detection signal DS1 is on low voltage level, phase comparator 8 generates an adjusting signal PH of low voltage level L. When both of drive signal D1 from drive circuit 7 and detection signal DS1 from limiter 61 have high or low voltage level or when one of drive signal D1 and detection signal DS1 has high voltage level and the other has low voltage level, phase comparator 8 generates an adjusting signal PH of intermediate level M.
Integrating circuit 57 averages outputs from phase comparator 8 to provide impedance regulator 40 with the averaged output. Accordingly, with faster phase of detection signal DS1, phase comparator 8 generates adjusting signal PH of high voltage level H to lower impedance of FET 44 in impedance regulator 40. Then, a large amount of electric current flows through FET 44 and resistor 45 to ground to elevate voltage on resistor 37 so that drive circuit 7 reduces the oscillation frequency to diminish drive frequency of IGBTs 11 and 12. To the contrary, with faster phase of drive signal D1, phase comparator 8 generates adjusting signal PH of low voltage level L to increase impedance of FET 44 in impedance regulator 40. Then, a small amount of electric current flows through FET 44 and resistor 45 to ground to reduce voltage on resistor 37 so that drive circuit 7 increases the oscillation frequency to augment drive frequency of IGBTs 11 and 12.
In this way, upper and lower limits of oscillation frequency in drive circuit 7 and oscillation pulses issued from drive circuit 7 are determined dependent on the value of voltage on resistors 37, 45 and 46 in impedance regulator 40. Drive circuit 7 varies oscillation frequency of drive signals D1 and D2 in response to level of adjusting signal PH from phase comparator 8 and produces drive signals D1 and D2 of varied oscillation frequency to IGBTs 11 and 12.
When AC power source 1 produces the output around zero voltage, input power to inverter circuit 3 comes to zero voltage accordingly, and simultaneously, high frequency AC power from inverter circuit 3 to heating coil 4 approaches zero voltage. This causes resonance waveform detector 6 to produce to phase comparator 8 detection signal DS1 of lowered voltage level below operation threshold value VTH for phase comparator 8 which therefore may fail to perform normal operation accompanied by abnormal oscillation in drive circuit 7.
FIG. 8 indicates waveforms of electric current and voltage at selected positions in induction heating apparatus shown in FIG. 6. During the period T of time shown in FIG. 8(a), approximately zero voltage of AC power source 1, results in reduction in amplitude of resonance current IL flowing through heating coil 4, and as shown in FIG. 8(b), resonance waveform detector 6 provides first input terminal IN1 of phase comparator 8 with detection signals DS1 of reduced voltage. Accordingly, when resonance waveform detector 6 generates detection signal DS1 of lowered voltage below operation threshold value VTH of phase comparator 8, it cannot produce adjusting signal PH in response to phase difference between detection signal DS1 from resonance waveform detector 6 and drive signal (oscillation pulse) D1 from drive circuit 7.
In this view, Japanese Patent Disclosure No. 6-176862 discloses an induction heating cooker which comprises a self-excitation oscillator for producing oscillation pulses as drive signals to a switching element, a comparative voltage detector for producing detection signals in response to electric power supplied from a rectifying circuit to an inverter circuit, a resonance voltage detector for producing detection signals in response to resonance voltage applied from inverter circuit to a heating coil, and a comparator for producing to self-excitation oscillator output signals in response to differential voltage between detection signals from comparative voltage detector and resonance voltage detector. As induction heating cooker of this reference adds voltage from a circuit power source to detection signal from comparative voltage detector through a waveform shaper, comparative voltage detector produces to comparator detection signals which are not lowered below operation threshold value of comparator even when AC power source produces approximately zero voltage to prevent comparator from producing abnormal trigger pulses to self-excitation oscillator. In this case, comparator does not produce also normal trigger pulses, however, self-excitation oscillator oscillates with the natural frequency to prevent abnormal oscillation of self-excitation oscillator which may produce abnormal drive signals to switching element.
Induction heating cooker of the reference, however, has a defect of performing abnormal operation. Specifically, while a control circuit promptly responds to existence or absence of or alteration in a heated object, self-excitation oscillator oscillates with the natural frequency, and when the natural frequency by self-excitation oscillator is rapidly and increasingly deviated from oscillation frequency by self-excitation oscillator driven by trigger pulses of comparator, drive circuit may disadvantageously supply control terminal of switching element with abnormal drive signals.
Therefore, an object of the present invention is to provide an induction heating apparatus capable of always stably turning a switching element of an inverter circuit on and off even during the period at which electric power produces the output of lowered voltage level. Another object of the present invention is to provide an induction heating apparatus capable of preventing rapid change in oscillation frequency of a drive circuit even when a control circuit promptly responds to change in a load.